1. Field of the Invention
This invention relates to a process of producing zeolite substantially free from a free alkali ingredient. More particularly, it relates to an improvement in a process wherein a silica source and an alumina source is reacted in an aqueous reaction medium containing an excessive amount of an alkali ingredient to form zeolite, followed by separating the so formed zeolite from the aqueous alkaline reaction medium or from an aqueous medium in which the zeolite, after being separated from the aqueous alkaline reaction medium, has been slurried.
Zeolite is an alkali metal or alkaline earth metal salt hydrate of an alumino silicic acid and is of various crystal forms. Generally, zeolite is used in the form of a dried finely divided powder or a pellet prepared from the powder. Zeolite possesses numerous micropores in which crystal water is retained. The crystal water is readily desorbed therefrom without destruction of the crystal lattice when zeolite is heated to an elevated temperature or placed under a reduced pressure. When the water-desorbed zeolite is exposed to moisture of a high humidity, it adsorbs water and reverts to the original state containing crystal water. Similar desorption-adsorption phenomena are also observed with respect to molecules other than water, provided that the molecules are of a size suitable for being entrapped by the micropores of zeolite. In addition, zeolite exhibits a catalyst activity for a particular reaction, for example, isomerization of xylene. Furthermore, zeolite possesses an ion exchange capability. In view of the above-mentioned properties, zeolite has various industrial applications, such as a drying agent, catalyst, catalyst support, adsorbent and ion exchanger.
2. Description of the Prior Art
In a typical wet type method of producing zeolite, a silica (SiO.sub.2) source and an alumina (Al.sub.2 O.sub.3) source are reacted with each other in an aqueous reaction medium containing an excessive amount of an alkali ingredient. The zeolite, so formed, is fine crystalline particles dispersed in the aqueous alkaline reaction medium and in the form of a slurry. The crystalline zeolite particles are separated from the aqueous alkaline reaction medium, and then, the separated zeolite is usually washed with water or neutralized to remove the alkali ingredient retained in the separated zeolite. Thereafter, the zeolite is dried and then optionally calcined, to obtain a final zeolite product in the form of a finely divided powder or a pellet. The separation of fine zeolite particles from the aqueous alkaline reaction medium or from the aqueous washing medium or netrualized medium is effected usually by means of centrifugation or filtration.
Such separation of fine zeolite particles in the conventional method of producing zeolite has some disadvantages. That is, first, since the zeolite particles are very fine, it is cumbersome and requires a substantial period of time to complete the separation. Secondly, the zeolite cake obtained by centrifugation or filtration of the aqueous zeolite slurry usually contains a salient amount, i.e, 30 to 70% by weight, of water, and when the cake is subject to an external force, it collapses and becomes highly viscous and oily. Thus, it is very cumbersome to completely recover the fluidized cake from the centrifugation or filtration apparatus. Furthermore, when the fluidized cake is dried, it is liable to undesirably adhere to the wall of a drying apparatus.